Accumulation of cholesterol in the vascular wall is believed to be a key event in the development of atherosclerosis (for review, see Ross, Nature 362: 801-809, 1993). Early in the development of atherosclerotic lesions, the artery walls are penetrated by cholesterol-containing atherogenic particles. These particles are believed to be derived from low-density lipoproteins (LDL) or to be minor subpopulations of sterol-rich lipoproteins. They are recognized as foreign by macrophages, which pick up the particles, integrate the cholesterol, and thereby become foam cells. Accumulation of foam cells is the first stage of fatty streak formation. These fatty streaks then enlarge through the gradual accumulation of lipid-containing macrophages and smooth muscle cells, eventually developing into fibrous plaques rich in smooth muscle cells.
The structure and activity of plasma lipoproteins have been reviewed by Gotto et al. (Meth. in Enzymology 128: 3-41, 1986). High-density lipoproteins (HDL) are a class of plasma lipoproteins which consist of cholesterol, cholesteryl esters, phospholipids, triglycerides, and apolipoproteins (principally apoA-I and apoA-II). HDL have been implicated in the transport of cholesterol (principally in the form of cholesteryl esters) from peripheral tissues to the liver, where the cholesterol is catabolized or excreted (Glomset, Lipid Res. 9: 155-167, 1968). The transfer of cholesteryl ester (CE), triglycerides (TG), and phospholipids (PL) is mediated by at least two different lipid transfer proteins, lipid transfer protein (LTP-I) (Nishide et al., J. Lipid Res. 30:149-158, 1989), also called cholesteryl ester transfer protein (CETP) (Drayna et al., Nature 327:632-634, 1987) and phospholipid transfer protein (PLTP), also called lipid transfer protein II (LTP-II) (Albers et al., Arteriosclerosis 4:49-58, 1984; Tollefson et al., J. Lipid Res. 29:1593-1602, 1988 and are incorporated herein in their entirety).
Several lines of evidence have strongly implicated CETP and PLTP as key regulators of HDL and LDL metabolism, controlling cholesterol homeostasis and the development of atherosclerosis. Studies with transgenic animals show that increased levels of CETP produce an atherogenic lipoprotein profile (increased LDL and decreased HDL) and are positively correlated with LDL cholesterol concentration and the degree of coronary artery atherosclerosis (Quinet et al., J. Clin. Invest. 87:1559-1566, 1991; Marotti et al., Nature 364:73-75, 1993). Humans with a genetic deficiency of CETP have no evidence of premature atherosclerosis and have significantly increased levels of HDL as well as reduced LDL (Inazu et al., New Eng. J. Med. 323:1234-1238, 1990). Evaluation of CETP-deficient individuals indicates a lipoprotein profile that is anti-atherogenic and associated with an increased life span. Together these studies provided convincing evidence that CETP is a proatherogenic protein.
Evidence suggests that CETP and PLTP act together to effect lipid transfer. CETP activity is enhanced by PLTP (Tollefson et al., ibid.) or by enriching HDL with phospholipid (Tall, J. Lipid Res. 27:361-367, 1986). Lipid transfer inhibitor protein modulates the activity of both transfer proteins (Nishide et al., J. Lipid Res. 30:149-158, 1989). Where earlier data have suggested that CETP alone was responsible for HDL interconversion (Lagrost et al., J. Lipid Res. 31:1569-1575, 1990), recently it has been shown that PLTP promotes the conversion of high density lipoproteins (HDL) into populations of larger and smaller particles in the absence of other lipoproteins (Jauhiainen et al., J. Biol. Chem. 268:4032-4036, 1993). However, while it was believed that PLTP and CETP both played roles in regulation of HDL and LDL, it appeared that there was little homology between various lipid transfer proteins (Tollefson et al., ibid. 988). The ability of PLTP to promote the conversion of HDL indicated that PLTP and CETP may act through synergistic mechanisms.
In view of the relationship between cholesterol and phospholipid transfer to cholesterol homeostasis and atherosclerosis, there is a need in the art for agents that regulate cholesterol homeostasis. However, elucidation of the interaction between CETP and PLTP in LDL and HDL phospholipid transfer has been impeded by the inability to isolate sufficient quantities of PLTP because PLTP is present only in trace amounts in vivo. Cloning of PLTP provides a means to produce larger amounts of recombinant protein. There is also a need in the art for agents that regulate levels of HDL, LDL and VLDL. In vitro measurements of components involved in phospholipid transfer are needed as diagnostics in both research and clinical settings. It is an object of the present invention to provide such agents. It is a further object of the invention to provide methods for controlling phospholipid transfer from LDL to other lipoprotein classes, and to provide pharmaceutical compositions for use within these methods. Towards these ends, the present invention provides novel polynucleotide molecules encoding a phospholipid transfer protein.